Process for conveyance of powder materials in hyperdense phase applicable to bypassing obstacles

ABSTRACT

Process for conveyance of a powder material in a hyperdense bed to bypass an obstacle, in which an obstacle bypass device including at least upstream, intermediate and downstream caissons is inserted between two horizontal conveyors, an upstream conveyor, and the downstream conveyor. At a device entry, at the upstream caisson lower duct is supplied with gas at pressure P 1,  the upstream caisson further including an upper duct which is a column. The intermediate caisson is between the upstream caisson and the downstream caisson at a level that bypasses the obstacle, a lower duct of which is fed with gas at pressure P 3  and an upper duct of which is connected to the upper duct of the upstream caisson and to an upper duct of the downstream caisson. At a device outlet, at the downstream caisson, a lower duct is supplied with gas at pressure P 2.  The three caissons remain full of powder material kept in a potential fluidization state at all times, with a pressure difference P 1− P 2  being kept strictly positive.

BACKGROUND OF THE INVENTION

The invention relates to an improvement to the powder materialconveyance and distribution system in hyperdense phase. This improvementmakes it possible to equip existing industrial installations with thishigh performance and economic conveyance system.

It is a continuous process for conveyance of a powder product in orderto feed a large number of packaging assemblies such as bagging machines,containerization devices, or a large number of production assembliessuch as plastic extruding presses or igneous electrolysis vat cells,from a single storage area.

Powder materials to be conveyed can be fluidized; their size grading andcohesion are such that injecting gas into them at low velocity caneliminate cohesion between particles and reduce internal frictionforces. For example, this type of material includes alumina for igneouselectrolysis, cements, plasters, quick lime or slaked lime, fly ash,calcium fluoride, magnesium chloride, all types of fillers for mixes,catalysts, coal dust, sodium sulfate, phosphates, polyphosphates orpyrophosphates, plastics in powder form, food products such as powdermilk flour, etc.

DESCRIPTION OF THE RELATED ART

Many devices have been studied and developed for conveyance of powdermaterials in fluidized bed. One particular problem is related to thecontinuous feed of the powder material regulated as a function ofconsumption requirements of the said material. One of the many examplesillustrating this problem is feed of alumina to igneous electrolysiscells for the production of aluminum.

In order to do this, the alumina, which is a powder product conveyed andsolubilized in the electrolytic bath, is consumed gradually whileelectrolysis is taking place, and must be replaced as it is consumed sothat the concentration of solubilized alumina remains optimum,encouraging maximum efficiency of the electrolysis cell. It then becomesnecessary to adjust the quantity of alumina added into the electrolysisvat, so that its operation is not disturbed by excess or insufficientalumina.

The powder materials conveyance device developed by the applicant anddescribed in European patents EP-B-0 122 925, EP-B-0 179 055, EP-B-0 187730, EP-B-0 190 082 and EP-B-0 493 279 enables continuous feed of powdersolids in their hyperdense phase. It is used particularly for regularand continuous feed to storage and distribution hoppers located in thesuperstructure of electrolytic pots.

This device comprises at least one horizontal conveyor called theair-pipe between the storage area and the area to be supplied, composedof a lower duct in which gas circulates, and an upper duct in which thepowder material is conveyed, the two channels being separated by aporous wall. Gas is blown into the lower duct through at least onesupply tube. The powder material completely fills the upper duct of theconveyor and this conveyor is fitted with at least one balancing columnpartially filled with powder material, the filling height balancing thegas pressure. This balancing column creates the conditions for potentialfluidization of the powder material. The powder material, which is notdisturbed very much due to the very low gas flow, is present in the airpipe in the form of a hyperdense bed.

In order to make the description of potential fluidization easier tounderstand, it is worth while repeating the principles of conventionalfluidization, normally used for conveying powder materials and describedfor example in patent U.S. Pat. No. 4,016,053. The device used influidization also comprises an air pipe as described above. Thefluidization gas is injected into the lower duct at a given pressurep_(f), passes through the said porous wall and then passes between theparticles at rest in the powder material forming the layer to befluidized. Unlike the potential fluidization device, the thickness ofthis layer at rest is very much less than the height of the upper ductof the said conveyor, in other words in the absence of any injection offluidization gas, the powder material only very partially fills theupper duct of the horizontal conveyor.

In conventional fluidization, by imposing a high gas flow, the saidparticles are moved and lifted, each of them losing its permanentcontact points with its neighbors. In this way the interstitial spacebetween the particles increases, internal friction between particles isreduced and these particles are put into a state of dynamic suspension.Consequently, the result is an increase in the initial volume of thepowder material and a corresponding reduction in the apparent density.

The term “dense phase” is usually reserved for pneumatic transport athigh pressure. The hyperdense phase is characteristic of potentialfluidization. To give an idea of the situation, consider the example ofthe case of alumina Al₂O₃ in which the solid/gas ratio is of the orderof 10 to 150 kg Al₂O₃/kg of air in dense phase pneumatic transport andis 750 to 950 kg Al₂O₃/kg of air for conveyance by potentialfluidization in the hyperdense phase. Therefore, the solid powder can beconveyed at very high solid gas concentrations in the hyperdense phase,significantly higher than the dense phase in pneumatic transport.

In the case of potential fluidization, even if no gas is injected, thepowder material almost completely fills the conveyance device andparticularly the upper duct. When gas is injected into the lower duct,the balancing column is partially filled with powder material occupyingthe upper duct at a manometric head that balances the pressure p_(f) andprevents the size of the interstices between the particles fromincreasing. Consequently, the presence of the balancing column preventsfluidization of the powder material present in the horizontal conveyorand forces the said material to appear as a hyperdense potentialfluidization bed. Furthermore, since the interstitial distance betweenparticles does not increase, the permeability of the medium to gasinjected at pressure p_(f) is very low and limits the gas flow to a verysmall quantity. We will subsequently refer to this low gas flow thatpasses through the balancing column “degassing”.

Therefore, no fluidization takes place, but it is possible to talk aboutpotential fluidization; there is no permanent circulation of material inthe air pipe, but flow will take place by successive collapsing as soonas the need for any powder material arises, for example when the levelof the area to be supplied drops below a critical value. When continuousconsumption of the material stored in the area to be supplied is suchthat the material level drops below the level of the orifice in thesupply pipe, a certain quantity of powder material will escape from thepipe creating a “vacuum” which will be filled by falling material, whichwill create a domino effect and thus continue throughout the air pipeworking backwards towards the storage silo.

The potential fluidization device for conveyance in a hyperdense bed, asdescribed in European patents EP-B-0 122 925, EP-B-0 179 055, EP-B-0 187730, EP-B-0 190 082 and EP-B-0 493 279, is used on a large scaleparticularly to supply 300000 ampere vats in recent installationsdesigned for igneous electrolysis of aluminum. The advantages of thisdevice are well known:

continuous feed to vats in order to keep the hoppers full at all times,

low system maintenance, low wear due to the low product circulationvelocity,

no size grading segregation,

low energy consumption,

perfect control over transport of the alumina, with no preferential blowoff.

In an electrolysis workshop, the number of areas to be supplied from asingle storage area may be high (several tens) and the distance betweenthe storage area and the area to be supplied may be high (severalhundreds of meters). The device illustrated in EP-B-0 179 055 iscomposed of a series of conveyors in cascade; a primary conveyor betweenthe storage area and a series of secondary conveyors, each assigned to apot and equipped with side take-off points that feed hoppers integratedinto the vat superstructure.

But this system imposes the use of horizontal or slightly inclinedconveyors, so that the sequence of small collapses (that occurprogressively in the air pipe as far as the storage silo) can occurunder optimum conditions. The applicant observed firstly that it isimpossible to keep the material in a state of potential fluidization ifthe conveyor is inclined at a steep slope and secondly that a suddenchange in the slope interrupts the “domino” effect of small collapsesand causes the formation of solid plugs in which the powder material canno longer be kept in the potential fluidization state.

However, an old workshop was not necessarily designed to be fed only byhorizontal or slightly inclined conveyors. There are sometimespassageways inclined conveyors. There are sometimes passageways reservedfor electrolysis service vehicles (liquid bath transport, metaltransport, etc.) and obstacles that conveyors cannot bypass to the leftor to the right, and where a level change is unavoidable.

Consequently, the applicant attempted to develop a process that wouldmake it possible to use the hyperdense phase conveyance system describedin European patents EP-B-0 122 925, EP-B-0 179 055, EP-B-0 187 730,EP-B-0 190 082 and EP-B-0 493 279, even for the purposes of renovatingequipment in old installations.

SUMMARY OF THE INVENTION

The process according to the invention is a process for conveyance ofpowder materials by potential fluidization capable of bypassingobstacles by changing levels, in other words releasing the hyperdensebed conveyance system from the constraint of using only horizontal orslightly inclined conveyors. These conveyors are qualified as“horizontal” in the rest of this description, even if they are slightlyinclined, for simplification purposes.

According to the invention, a device to bypass an obstacle comprising atleast three caissons is inserted into the hyperdense bed conveyor systemadjacent to the obstacle to be bypassed between two horizontal conveyors(one will be called the “upstream” conveyor and the other the“downstream” conveyor):

at the entry to the device, an upstream caisson comprising a lower ductcontaining gas fed at pressure P1 and an upper duct or pipe composedessentially of a column connected at one end to the upper duct of theupstream conveyor and at the other end to the upper duct in theintermediate caisson;

in the middle, at a level that goes above the obstacle, at least oneintermediate caisson comparable to a horizontal air pipe, the lower ductof which is fed with gas at pressure P3 and the upper duct of which isconnected through its first end to the upper duct of the upstreamcaisson, is connected at its second end to the upper duct of thedownstream caisson;

at the outlet from the device, a downstream caisson comprising a lowerduct supplied with gas at pressure P2 and an upper duct or pipe composedessentially of a column connected firstly to the upper duct of theintermediate caisson and secondly to the upper duct of the downstreamconveyor.

The obstacle is at the same level as at least one of the horizontalconveyors and the intermediate caisson is not at the same level as theobstacle, so that it can bypass it. The horizontal conveyors are usuallyat the same level, but there is no reason why there should not be adifference in height between these two horizontal conveyors. Theintermediate caisson is long enough to get past the obstacle to thepowder material to be conveyed.

The particular feature of the device is that it creates a pressuredifference ΔP=P1−P2 which is always strictly positive, the pressuredifference being such that the three caissons remain full of powdermaterial kept in a potential fluidization state at all times. By makingsure that this pressure difference remains positive, the device actslike a hydraulic siphon; note that the product flow takes place freely,continuously and regularly from the first horizontal conveyor to thesecond.

Preferably, the columns on the upstream caisson and the downstreamcaisson are balancing columns full of powder product at a height suchthat the free level of the said material in each of these columns is ator above the highest point in the air pipes belonging to the groupconsisting of the intermediate caisson and the parts of the upstream anddownstream conveyors located close to their junctions with the bypassdevice. Since the pressure difference ΔP=P1−P2 is always positive and isguaranteed when the height of the powder material in the column of theupstream caisson is greater than the height of the powder material inthe column of the downstream caisson.

In practice, the device according to the invention applied to theconveyance of alumina preferably uses an intermediate caisson located ata lower level than the two horizontal conveyors, so that alumina can gounder the floor of the vats to leave free passage for electrolysisservice vehicles. But a passage above or at an intermediate level wouldalso be possible. The essential point is firstly to make sure that thefree alumina level in the two columns is at the highest point of the twoconveyors and the intermediate caisson, and secondly that the aluminaheight in the first column is greater than the alumina height in thesecond column.

The pressure in the intermediate caisson is used to put the powdermaterial into a potential fluidization state. Preferably, its value isintermediate between the potential fluidization pressure of the firstcolumn and the potential fluidization pressure of the second column.

If the system is to operate correctly, it is useful to form a spacewithout any powder material in the high part of the intermediate caissonupper duct, forming a pressurized gas bubble. The applicant has observedthat in general, the presence of gas bubbles in the high part of upperducts in hyperdense phase conveyors improves potential fluidizationconditions and enables better circulation of the fluidization gas. TheFrench patent application FR 9806124 deposited by the applicant on May11, 1998 describes devices adapted to the creation of bubbles stable inthe high part of the upper ducts of conveyors. In fact, it is sufficientto extend each column by a penetration on each side of the high part ofthe intermediate caisson upper duct. The height of the penetration ispreferably between one half and one hundredth of the height of theuseful part of the air pipe conveying the powder material.

BRIEF DESCRIPTION OF THE DRAWINGS

The process according to the invention will be better understood afterreading the detailed description of the various devices described belowusing non-limitative examples.

FIG. 1 shows a diagrammatic vertical section through a first deviceaccording to the invention, which bypasses an obstacle by going underit.

FIG. 2 is a diagrammatic vertical section through a second deviceaccording to the invention, used for extraction of powder material froma silo located close to an obstacle.

FIG. 3 is a diagrammatic vertical section through a third deviceaccording to the invention, which bypasses an obstacle by going over it.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

The Hyperdense Siphon (FIG. 1)

This example is taken from an aluminum plant that is being modernized byadopting a continuous alumina feed system and in which, starting from asilo located above the level of the vats, the alumina conveyor has to bepassed under the floor of the vats to leave space to allow electrolysisservice vehicles to pass (transport of liquid bath, metal, etc.).

The bypass device is designed and made in accordance with thecharacteristics of the process according to the invention. In thisparticular case, the bypass goes under the vat. Although the mediumbeing circulated (alumina+gas) is compressible in this case and theconcept of a siphon applies to hydrodynamic flows only, it is impossibleto avoid seeing an analogy between the device in this example shown inFIG. 1 and a hydraulic siphon, due to its shape and the function that itfulfills. This is why the applicant has called it “hyperdense siphon”.

The applicant has also observed that no solid plug is created with thistype of device; the product is kept in a state of fluidization orpotential fluidization at all points and free circulation of the productis achieved regardless of the passage through a low point.

The said hyperdense siphon comprises three distinct potentialfluidization caissons:

at the inlet, a caisson S1 comprising a lower duct S1.1 supplied withgas at pressure P1 and an upper “duct” S1.2, consisting essentially of acolumn C1 connected at one end to the upper duct T1.2 of the upstreamconveyor T1 and at the other end to the upper duct S3.2 of theintermediate caisson S3;

in the middle, at a level which can bypass the obstacle, an intermediatecaisson S3 comparable to a horizontal air pipe, in which the lower ductS3.1 is supplied with gas at pressure P3 and in which the upper ductS3.2, connected at its first end to the upper duct S1.2 of the upstreamcaisson S1, is connected at its second end to the upper duct S2.2 of thedownstream caisson S2;

at the outlet from the device, a downstream caisson S2 comprising alower duct S2.1 supplied with gas at pressure P2 and an upper “duct”S2.2 consisting essentially of a column C2 connected at one end to theupper duct S3.2 of the intermediate caisson S3 and at the other end tothe upper duct T2.2 of the downstream conveyor T2.

The horizontal conveyors T1 and T2 are at the same level in this case,but there is no reason why the upstream conveyor and the downstreamconveyor should not be at a different height.

The length L of the intermediate caisson, equal to 20 meters, issufficient in this case to take the powder material to be conveyedbeyond the obstacle. If a greater length L is necessary, it ispreferable to connect the caisson S3 with other intermediate caissonsS3, S″3, etc., identical to S3, such that they have a common upper ductand lower ducts supplied with gas at a potential fluidization pressureP3, P″3, etc.

Column C1 is filled with alumina over a height h1 such that the freelevel of the said material 15.1 is higher than the highest point of theair pipes T1, T2 and S3. Similarly column C2 is filled with alumina overa height h2 such that the free level of the said material 15.2 is alsohigher than the highest point of the air pipes T1, T2 and S3. Thepressure difference ΔP=P1−P2 is always positive and is achieved when theheight of the powder material h1 is kept greater than h2.

The intermediate caisson S3 is lower than the caisson on the twohorizontal conveyors T1 and T2; the distance h0 is about 6 meters.Therefore in order to leave free passage for electrolysis servicevehicles, it was necessary to pass the alumina 6 meters below the levelof the main conveyor over a distance of 20 meters, and then to lift itup by about 6 meters again, due to the vat layout.

The pressures P1 and P2 are adjusted such that the system remains fullof alumina at all times. The pressures P1 and P2 are such that:

P 1=h 1*d 1

and

P 2=h 2*d 2,

where d1 is the average density of the product in potential fluidizationin column C1, and d2 is the average density of the product in potentialfluidization in column C2. The density of the fluidized product variesfrom one column to another; it is lower when the fluidization pressureis higher.

The applicant has observed that all that is necessary is to make surethat height h1 is greater than height h2, so that pressure P1 will begreater than P2 and thus the device will operate like a hydraulicsiphon; there is a natural flow of alumina despite the low point imposedby the geometry of the device.

The pressures are chosen to be equal to the following values:

P1=0.7 bars−P2=0.6 bars−P3=0.65 bars.

It is then found that the level h1 of the product with medium density0.85 is about 8.2 meters, whereas h2 is about 7 meters, and the productflows naturally through the hyperdense siphon, going down through columnC1, following caisson S3 and rising again through column C2.

In order for this system to work properly, a bubble B of gas was formedin the upper part of the intermediate caisson S3. This gas bubble isobtained conventionally by penetration of columns C1 and C2 into theupper part of the intermediate caisson S3.

EXAMPLE 2

Extraction at the Silo Bottom (FIG. 2)

Another application of the siphon in the hyperdense phase is extractionof a powder product from the bottom of a silo.

The silo is not always located very close to the ground. In this case itis necessary to raise the product after it is extracted, for subsequentuse at a level higher than the level of the bottom of the silo, forexample to feed other conveyor equipment without the need to make areclaim pit for the product to be conveyed.

FIG. 2 shows the diagram of such a device. Silo 1 feeds the siphonconsisting of the bottom 2 of silo 1, the intermediate conveyor 3 andcolumn C. The siphon itself feeds the horizontal conveyor T or any otherhandling or storage system.

The intermediate conveyor 3 is composed of a lower duct 6 and an upperduct 7, connected at one end to column C, and at the other end to silo 1through an area 4 at the lower part of bottom 2 of silo 1. A gas G isinjected through a tube 8 at a pressure P. This gas passes through theporous wall 5 that separates the lower duct 6 and the upper duct 7.

In this configuration, silo 1 is an overhead storage tank; it is notfluidized. The height H representing the alumina head in silo 1 must begreater than the elevation height h of the product 12. The lower part 4of the bottom 2 of silo 1 is then in a potential fluidization condition,so that the powder material 12 can flow well through the siphonconsisting of bottom 2 of silo 1, the intermediate conveyor 3 and columnC.

If the system is to operate properly, it is advantageous to form a spacefree of powder material in the high part 14 of the upper duct 7 of theintermediate caisson, forming a pressurized gas bubble B. A stablebubble B can be created by extending column C with a penetration 40.1,or by extending bottom 2 of silo 1 with a penetration 40.2.

In this example, the fluidization pressure is such that P=d*h, where dis the average density of the solid when in the fluidization state.

It has been shown experimentally that for alumina, it is quite possibleto lift the product to a height h equal to 7 m, by imposing an airpressure P of the order of 0.6 bars.

Obviously, these values are not restrictive and the pressure P can beincreased in order to raise the product to the required height.

EXAMPLE 3

Bypass Over the Top (FIG. 3)

FIG. 3 illustrates a device used to bypass an obstacle by going over it.The various parts of the device are marked with the same references asthose used in the example in FIG. 1 and in FIG. 2.

This type of device cannot operate in isolation. It is connected to theoverhead storage tank by means of a set of air pipes represented in FIG.3 by the air pipe T. As in the example 2, the upper level of alumina inthis overhead storage tank 1 is often higher than the free levels ofalumina in the columns of the bypass device, and particularly theupstream column (the height H of alumina must then be greater than theheight h in column C1).

Advantages of the Process According th the Invevtion

This process is used to design and make hyperdense siphons used:

to supply electrolysis vats between two horizontal conveyors, passingunder or over a free passage;

or for extraction from a silo when the silo is located close to theground.

What is claimed is:
 1. Bypass device (10, 30) for conveying a powdermaterial in a hyperdense phase and connected to an upstream conveyor(T1) and a downstream conveyor (T2), comprising at least upstream,intermediate and downstream caissons: the upstream caisson (S1)comprising a lower duct (S1.1) supplied with gas at pressure P1 and anupper duct (T1.2) comprising a column (C1), connected at one end to anupper duct (T1.2) of he upstream conveyor (T1) and at the other end toan upper duct (S3.2) of the intermediate caisson (S3); at least one saidintermediate caisson (S3), having a lower duct (S3.1) supplied with gasat pressure P3 and an upper duct (S3.2) connected at a first end to theupper duct (S1.2) of the upstream caisson (S1), and connected through asecond end to an upper duct (S2.2) in the downstream caisson (S2); thedownstream caisson (S2) comprising a lower duct (S2.1) supplied with gasat pressure P2 and an upper duct (S2.2) comprising a column (C2)connected at one end to the upper duct (S3.2) of the intermediatecaisson (S3) and at another end to an upper duct (T2.2) of thedownstream conveyor (T2).
 2. Bypass device (10) according to claim 1,wherein the intermediate caisson (S3) is lower than the upstreamconveyor (T1) and the downstream conveyor (T2).
 3. Bypass device (30)according to claim 1, wherein the intermediate caisson (S3) is higherthan the upstream conveyor (T1) and the downstream conveyor (T2). 4.Bypass device (10, 30) according to claim 1, wherein the column (C1) andthe column (C2) are balancing columns filled with powder material over aheight (h1, h2) such that a free level (15.1, 15.2) of the material ineach of the columns is above the highest point in air pipes selectedfrom the group consisting of the intermediate caisson (S3) and parts ofthe upstream conveyor (T1) and downstream conveyor (T2) located close totheir junctions with the bypass device.
 5. Bypass device (10, 30)according to claim 1, wherein the pressure P3 in the intermediatecaisson (S3) is intermediate between a potential fluidization pressureP1 in the first column (C1) and a potential fluidization pressure P2 inthe second column (C2).
 6. Process for conveyance of a powder materialin a hyperdense bed to bypass an obstacle, in which an obstacle bypassdevice (10; 20; 30) comprising at least upstream, intermediate anddownstream caissons is inserted between two horizontal conveyors, anupstream conveyor (T1), and a downstream conveyor (T2), comprising thesteps of: at a device entry, at the upstream, caisson (S1) supplyingwith gas at pressure P1 a lower duct (S1.1), the upstream caissonfurther comprising an upper duct (S1.2) comprising a column (C1)connected at one end to the upper duct (T1.2) of an upstream conveyor(T1) and at another end to an upper duct (S3.2) in the intermediatecaisson (S3); in said intermediate caisson between said upstream caissonand the downstream caisson at a level that bypasses the obstacle,feeding a lower duct (S3.1) with gas at pressure P3 and connecting anupper duct (S3.1) which is connected through a first end to the upperduct (S1.2) of the upstream caisson (S1), at a second end to an upperduct (S2.2) of the downstream caisson (S2); and at a device outlet, atthe downstream caisson (S2) supplying with gas at pressure P2 a lowerduct (S2.1), the downstream caisson further comprising an upper duct(S2.2) comprising a column (C2) connected at one end to the upper duct(S3.2) of the intermediate caisson (S3) and at another end to the upperduct (T2.2) of the downstream conveyor (T2); wherein said caissonsremain full of powder material kept in a potential fluidization state atall times, a pressure difference P1−P2 being kept strictly positive. 7.Process according to claim 1, wherein air pipes connect an overheadstorage tank (1) to an area to be supplied, and wherein a free level ofpowder material in the overhead storage tank (1) is higher than a freelevel of the powder material filling each column (C; C1; C2) of thebypass device (10; 20; 30).
 8. Process according to anyone of claim 1,wherein a space without any powder material is created in a high part(14) of the upper duct (S3.2) of the intermediate caisson (S3), forminga pressurized gas bubble (B).
 9. Process according to claim 8, whereinthe column (C1) of the upstream caisson (S1) and the column (C2) of thedownstream caisson (S2) are balancing columns.
 10. Process according toclaim 9, wherein powder material in column (C1) in the upstream caisson(S1) has a height (h1) which is kept greater than height (h2) of thepowder material in the column (C2) in the downstream caisson (S2). 11.Process according to claim 9, wherein the columns (C1) of the upstreamcaisson and (C2) of the downstream caisson are supplied with powdermaterial over a height (h1, h2) whereby a free level (15.1, 15.2) of thematerial in each of the said columns (C1, C2) above a highest point ofair pipes selected from the group consisting of the intermediate caisson(S3) and parts of the upstream conveyor (T1) and the downstream conveyor(T2) located adjacent to their junctions with the bypass device (10; 20;30).